Hydraulically operated impact devices are used for example in drilling machines designed for rock drilling and in different impact hammers designed for breaking rock, concrete and other similar hard materials. Such impact devices are usually arranged to a base machine, such as a movable carrier, and operated by the hydraulics of the base machine.
An impact device comprises a frame and a percussion piston reciprocated in relation to the frame by pressure liquid, compressed air or a similar pressure medium. The percussion piston delivers successive blows via a tool at the object to be handled. The pressure liquid is supplied to and from the percussion piston by means of suitable ducts. The percussion piston comprises working pressure surfaces and by varying the hydraulic pressure acting on the surfaces, the percussion piston is engaged in a reciprocating motion required by the operating cycle. Pressure liquid flows to the working surfaces of the percussion piston are typically controlled by means of different control slides. The control slides are moved by guiding a control pressure to act on the working pressure surfaces of the slides. Publication EP 0 426 928, for example, discloses a percussion hammer in which a sleeve-like control valve is arranged around a percussion piston, the control valve being arranged to open and close pressure fluid ducts connected to the working pressure spaces of the percussion piston. Control pressure is supplied from control pressure ducts to shoulders of the sleeve-like control valve to make the sleeve to move in a desired manner and to change the direction of motion of the percussion piston as required by the operating cycle. WO publication 99/54094 describes another solution in which a tube-like control slide is moved in a separate chamber by means of control pressure. The position of the control slide in the chamber defines the pressure fluid flows to the working pressure surfaces of the percussion piston. A common feature of current solutions is that the percussion piston comprises working pressure surfaces, such as shoulders, the motion of the percussion piston causing the surfaces to open and close high-pressure ducts formed in the frame of the impact device, return ducts leading to a tank and the control pressure ducts used for controlling the control slide. The control of the control slide depends on the travel of the percussion piston. The travel direction of the percussion piston can only be changed after the percussion piston has reached a predetermined position where it opens the control pressure conduit of the control slide and changes the position of the control valve. Due to their physical dimensioning, the operating cycles of known impact devices are thus based on fixed timing. Therefore the frequency and velocity of impact can be adjusted during drilling only by changing the impact pressure. A further drawback of known structures is that leakage gaps are fairly wide. Since the frame of the impact device is provided with control pressure ducts connected to the pressure spaces of the percussion piston for controlling the control slides, leakage of pressure medium from the gaps between the shoulders and the pressure spaces into the discharge duct takes placed during an operating cycle. The leakages add to the pressure medium consumption, which must be taken into account when the flow ducts and pumps of the pressure medium are being dimensioned. In addition, leakages naturally degrade the efficiency of the impact device.